The domestication of many plants has correlated with dramatic increases in yield. The identification of specific genes responsible for the dramatic differences in yield, in domesticated plants, has become an important focus of agricultural research.
One group of genes effecting yield are the nitrate reductase genes. These genes have utility for improving the use of nitrogen in crop plants, especially maize. The genes can be used to alter the genetic composition of the plants rendering them more productive with current fertilizer application standards, or maintaining their productive rates with significantly reduced fertilizer input. Increased nitrogen use efficiency can result from enhanced uptake and assimilation of nitrogen fertilizer and/or the subsequent remobilization and reutilization of accumulated nitrogen reserves. Plants containing these genes can therefore be used for the enhancement of yield. Improving the nitrogen use efficiency in corn would increase corn harvestable yield per unit of input nitrogen fertilizer, both in developing nations where access to nitrogen fertilizer is limited and in developed nations were the level of nitrogen use remains high. Nitrogen utilization improvement also allows decreases in on-farm input costs, decreased use and dependence on the non-renewable energy sources required for nitrogen fertilizer production, and decreases the environmental impact of nitrogen fertilizer manufacturing and agricultural use.
Many efforts have recently been made to improve nitrogen efficiency of crop plants through overexpression of nitrate reductase (NR) in plants. One group has applied expression of the Nicotiana plumbaginifolia & Arabidopsis thaliana NR cDNA or gene under control of different promoters such as 35S CaMV (Nicotiana plumbaginifolia) Ferrario, et al., (1995) “Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply” Planta 196:288-294; and Lhcb1*3::Nia1*2 (Arabidopsis thaliana) Nejidat, et al., (1997) “Increased protein content in transgenic Arabidopsis Thalina over-expressing nitrate reductase activity” Plant Science 130:41-49. (references). Although increases in NR expression levels up to 2-5-fold were detected with the 35S CaMV::Nia-2 gene in Nicotiana plumbaginifolia plants (Foyer, et al., (1994) “Adaptations of photosynthetic electron transport, carbon assimilation and carbon partitioning in transgenic Nicotiana plumbaginifolia plants to changes in nitrate reductase activity” Plant Physiol. 104:171-178), no improved nitrogen efficiency was observed. All these attempts to over express this enzyme have not resulted in improved growth under lower nitrogen fertility.
Therefore, despite several attempts to improve NR efficiency, no satisfactory composition or method has been provided that leads to an improvement of growth, productivity, and/or yield for agricultural crop plants. For these and other reasons, there is a need for the present invention.